16.3 - Fertilisation & Germination

Fertilisation in flowering plants

• Fertilisation happens after a pollen grain has landed on the stigma of a flower.
• It is the fusion of two nuclei:
  • The nucleus of the male gamete, carried in a pollen grain
  • The nucleus of the female gamete, held inside an ovule
• When these two nuclei fuse, they form a single cell called a zygote.
• The zygote then divides repeatedly by mitosis to form an embryo - the beginning of a new plant.

The structure of a seed and its food reserves

A seed has three main parts:

1. Embryo - The young plant, made up of an embryo root and an embryo shoot that grow into the adult plant
2. Food store - A supply of lipids and starch that the young plant lives on at first
3. Seed coat - A tough protective outer covering that keeps the inside safe until the seed germinates

Why the seed relies on its food store

1. At first the seed cannot photosynthesise - it has no green leaves and may be underground in the dark - so it lives entirely on its food store.
2. When the seed germinates, it draws glucose from its food store and respires it. This provides the energy and materials the seedling needs to grow its first root and shoot.
3. The seed keeps relying on its food store until the young plant has grown above the soil and produced green leaves.
4. Once it has green leaves, it can make its own glucose by photosynthesis and no longer depends on the food stored in the seed.

The conditions needed for germination

• Germination is the process by which a seed develops into a young plant.
• It's controlled by enzymes inside the seed.
• A seed often lies dormant (inactive) until conditions are right.
• Germination only begins when three conditions are all present together: water, oxygen, and warmth.
• If any one of these is missing, the seed stays dormant and does not germinate.

Why each condition is needed

• Water - Makes the seed swell and activates the enzymes inside it, which control the growth of the seedling.
• Oxygen - Needed for aerobic respiration, the process that provides the energy the seed needs to grow.
• Warmth (a suitable temperature) - Raising the temperature raises enzyme activity and so the rate of growth.
  • If it's too cold, the enzymes work too slowly.
  • Very high temperatures denature the enzymes and stop germination altogether.
  • The temperature that suits germination depends on the type of seed.